Structural material

ABSTRACT

For providing a structural material having improved fatigue strength and abrasion resistance, provided is a structural material containing iron and carbon, including: a first layer formed of pearlite; a second layer formed of a mixed phase of martensite and a carbide; and a third layer formed of a carbide, in order from a center to a surface of the structural material. The carbide of the third layer is represented by MC (where M is one element among Ti, V, Nb, Mo, Ta, and W), and the structural material has a concentration gradient in which an M element concentration is decreased from the surface to the center of the structural material.

TECHNICAL FIELD

The present invention relates to a surface modification method for increasing fatigue strength of low alloy steel and a structural material manufactured using the method.

BACKGROUND ART

As a method of controlling texture for improving mechanical properties, corrosion resistance, and functionality in an iron-based material, a surface hardening method has been reviewed. For example, PTL 1 discloses a rolling member manufactured using a surface hardening method.

CITATION LIST Patent Literature

PTL 1: JP 2006-009145 A

SUMMARY OF INVENTION Technical Problem

The rolling member of PTL 1 is provided with a first quenched hardened layer having martensite as a matrix phase; and a second quenched hardened layer formed in a deeper layer than the first quenched hardened layer, in which cementite is dispersed in a matrix phase containing at least one of a martensite phase and a bainite phase in which carbon is solid-solubilized. PTL 1 is aimed at strength improvement; however, particularly additional improvement in terms of fatigue strength and abrasion resistance is being required.

Solution to Problem

The present invention provides a structural material containing iron and carbon, including: a first layer formed of pearlite; a second layer formed of a mixed phase of martensite and a carbide; and a third layer formed of a carbide, in order from a center to a surface of the structural material. The carbide of the third layer is represented by MC (wherein M is one element among Ti, V, Nb, Mo, Ta, and W), and the structural material has a concentration gradient in which an M element concentration is decreased from the surface to the center of the structural material.

Advantageous Effects of Invention

By adopting a three-layer structure as such, a structural material having improved fatigue strength and abrasion resistance can be provided. By applying the structural material to a sliding material, or the like, a lighter weight, a longer lifespan, and improved reliability of the entire structural material can be achieved, as compared with the related art.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an optical micrograph of a section.

FIG. 2 is a Vickers hardness distribution.

FIG. 3 is an X-ray diffraction pattern.

FIG. 4 is a schematic diagram of a sectional texture.

FIG. 5 is a result of a fatigue test.

DESCRIPTION OF EMBODIMENTS

As a representative inexpensive structural material, low alloy steel can be mentioned. The low alloy steel is an iron carbon-based material having a content of transition metal elements such as Cr or Mo as an additive element of 20 wt % or less. The low alloy steel is a material having an A1 transformation point and having a carbide form being greatly different between a high temperature side and a low temperature side of the transformation point. That is, in a higher temperature side than the A1 transformation point, γ (austenite) or a carbide is grown, and in a lower temperature side than the A1 transformation point, a (ferrite) and a carbide can be formed, and a layered carbide is easily grown when being maintained in the low temperature side. Here, the carbide has M₃C as a main, wherein M is a transition metal element such as Fe, Cr, or Mo.

For spheroidizing of the carbide, the layered carbide is partially dissolved in a γ phase and aggregated, whereby braking of the layered carbide is progressed for surface energy reduction so that the layered carbide becomes closer to a sphere from an elliptical shape. Carbon other than the spheroidized carbide is solid-solubilized in γ. Pearlite is grown from the γ phase at an A1 point or less to form a layered carbide between carbides, and is further heated to a higher temperature side than the A1 point, whereby the layered carbide is broken and aggregated. Heating and cooling are repeated between the high temperature side and the low temperature side of the A1 point, thereby growing the spherical carbide. Then, the pearlite refers to a texture which is a layered structure and formed by alternating Fe(α-Fe) and Fe3C of a bcc structure.

The spherical carbide is grown as described above and the matrix is rapidly cooled in a temperature range in which the matrix is a γ phase, whereby the matrix can be transformed from the γ phase into a mixed phase of martensite and a residual γ phase. For forming MC (wherein M is at least one of Ti, V, Nb, Mo, Ta, and W) which is a high-hardness carbide, on the outside of a martensite layer, a slurry of powder containing an M element thereof is applied on a surface of a structural material at a thickness of 10 to 100 μm before a carburization process. The slurry is a mixed body of amorphous powder having a powder diameter of 10 μm or less and an alcohol solvent, and applied several times in the air. Thereafter, the solvent is evaporated by drying and heating, and heating is performed in a γ stable temperature range of the structural material. By the heating, some of M elements are diffused into a base material and forms a concentration gradient. Next, MC is formed by primary carburization and at the same time carbon is diffused also to a base material side.

The M element is diffused from the surface of the base material to about 10 μm, whereby close adhesion of MC growing later can be increased, and growth of γ crystal grains in the base material side by diffusion of the M element can be simultaneously suppressed, and a carbide containing the M element grows in a grain boundary and in grains to contribute to miniaturization of a lathe or a packet of martensite simultaneously to suppress crack propagation.

Use of nitrogen-containing gas during or after carburization allows nitrogen martensite and a carbon nitrogen compound to grow, thereby contributing to an increase of tempering softening resistance or prevention of deformation by quenching temperature lowering.

In the present invention, a layer constitution from the outermost surface to the inside of the structural material is changed as follows. On the outermost surface, an MC (wherein M is at least one of Ti, V, Nb, Mo, Ta, and W) or MCN-based carbonitride (wherein M is at least one of Ti, V, Nb, Mo, Ta, and W) is grown, and the thickness thereof is 10 μm or more. In order to increase close adhesion, a diffusion layer of the M element is formed in the inner side of this high-hardness carbide or a carbon nitrogen compound, and the matrix is martensite. In the martensite, a spherical carbide is dispersed and grows. The martensite is decreased toward a center part of the structural material, and becomes pearlite or a mixed phase of ferrite and pearlite. In the present invention, the carbide is dispersed in the martensite matrix, and the concentration of the M element contained in the carbide is high on the surface side and low in the center side. In addition, the texture of martensite becomes fine as the surface layer.

The life of sliding units may be greatly improved by forming the M element-containing carbide.

In Examples 1 to 4, an SCM420 material was used to describe the characteristics of the structural material; however, the effect of improving a lifespan can be obtained by including iron and a carbide.

Further, the SCM420 material is defined as having a composition of C: 0.18 to 0.23, Si: 0.15 to 0.35, Mn: 0.60 to 0.90, P: 0.030 or less, S: 0.030 or less, Ni: 0.25 or less, Cr: 0.90 to 1.20, and Mo: 0.15 to 0.25, according to the JIS specification, and in Examples 1 to 4, one alloy steel satisfying the range was selected and used.

Example 1

Ti is pulverized in a methanol solvent to form an amorphous form having a Ti particle diameter of 1 to 50 μm. Methanol containing 10 wt % of Ti pulverized powder is applied on the surface of an SCM420 material (0.2 wt % of C, 0.3% of Si, 0.7% of Mn, 0.1% of Cu, 0.1% of Ni, 1.1% of Cr, 0.2% of Mo, and the balance of Fe) in the air. The thickness of Ti after application is 200 μm. Methanol as a solvent is evaporated and dried, and inserted in a carburizing furnace. The inside of the carburizing furnace is vacuum-exhausted to 1 Pa and then substituted with Ar to clean the atmosphere in the furnace.

The partial pressure of H₂O and O₂ is decreased and whether the time change is minimized is confirmed, and then heating is started. Heating is performed to 1100° C. at a rate of 10° C./min and the temperature is maintained for 1 hour, and acetylene (C₂H₂) gas is introduced. Diffusion is performed from the surface to the inside of the structural material by the introduction of the acetylene gas. In addition, Ti on the surface of the structural material and a part of Ti diffused from the surface of the structural material to the inside are carbonized. By the carbonization, Ti becomes TiC or TiCN. The concentration of carbon is higher on the surface than in the inside. Ti is diffused at a temperature of 1100° C. and diffused into the grain boundary or into the grains of SCM420.

A part of diffused Ti forms TiC by the diffusion of carbon by acetylene introduction. The TiC does not continuously grow in the γ grain boundary of SCM420 and grows in a granular form. A total carbon supply amount at 1050° C. is 1.2 wt % in the SCM420 side in the interface between TiC and SCM420.

TiC and SCM420 carburized at 1100° C. are reheated to 850° C. and carburized again by acetylene gas. A total carbon supply amount at 850° C. is smaller than the supply amount at 1050° C., and the concentration of carbon in the SCM420 side in the interface between TiC and SCM420 is 1.4 wt %. By the carburization, a spherical carbide is grown in the grain. After the carburization, oil quenching and tempering are performed. An oil temperature is 100° C. and a tempering temperature is 160° C.

The texture of the structural material manufactured by the process has a three-layer structure in order from the center to the surface in a depth direction. A first layer is formed of pearlite, a second layer is formed of a mixture of martensite and a carbide, and a third layer is formed of a carbide. The first layer further includes ferrite, or may further include a mixture of pearlite and ferrite.

As compared with the case of carburization quenching and tempering without forming a carbide, a fatigue life measured by a rotating bending fatigue test is 20 times longer in the present Example. A factor causing the high fatigue life is represented below. TiC or TiCN is formed on the surface at a thickness of 200 μm and the hardness is 1500 to 3000 Hv. A region in which TiC or TiCN is dispersed in SCM420 is formed between SCM420 and TiC-based films, thereby increasing close adhesion of TiC or TiCN. Around the granular TiC, martensite and residual austenite are formed, and the hardness of martensite is 800 Hv. Granular TiC is not confirmed in the center of the material, and grown by diffusion of Ti and carbon, and an amount of TiC is decreased from the surface to a center part, and as the carbide Fe₃C-based is increased rather than TiC-based. By having the constitution, a high-hardness TiC or TiCN-based film which is difficult to be abraded is formed in the surface layer, and occurrence and propagation of cracks are suppressed by dispersed high-hardness TiC and fine martensite around the high-hardness TiC, and Fe₃C and martensite around the high-hardness TiC.

Example 2

On the surface of an SCM420 material (0.2 wt % of C, 0.3% of Si, 0.7% of Mn, 0.1% of Cu, 0.1% of Ni, 1.1% of Cr, 0.2% of Mo, and the balance of Fe), Al is deposited at a thickness of 50 μm, and Ti is plated by an exchange reaction of Al and Ti in a solvent containing Ti acetylacetate and fluorine. The SCM420 material on which the Ti plated film is grown is inserted to a carburizing furnace and heated to 1100° C. By heating to 1100° C., interdiffusion proceeds between Ti and SCM420, and Ti is diffused into the grain boundary or into the grain of SCM420 and partially becomes TiC. After Ti is diffused, the temperature is lowered to 1050° C. and acetylene is introduced. An amount of introduced acetylene depends on a depth and a texture requiring hardness (amount of carbide or the like); however, up to 1.2 wt % of carbon is introduced in a γ region. By the primary carburization, martensite is partially grown in the vicinity of the surface layer of SCM420 by nitrogen gas cooling. At this cooling rate, carbides more than 10 m do not grow in the grain boundary of SCM420.

After cooling, heating is performed to a temperature of an A1 point (codeposition transformation point) or more, and a carbide is precipitated in the γ crystal grain. From the precipitated carbide, carbon is diffused to the inside of SCM420, and acetylene is introduced for suppressing a decrease in a carbon concentration, thereby diffusing carbon from the surface. Due to the introduction of acetylene, the carbide is grown, so that a granular carbide is dispersed and grown in a γ crystal grain boundary or into the grain, and the particle diameter becomes 0.5 to 2 μm. Here, carbon is diffused into SCM420 through TiC.

The texture of the structural material manufactured by the process has a three-layer structure in order from the center to the surface in a depth direction. A first layer is formed of pearlite, a second layer is formed of a mixture of martensite and a carbide, and a third layer is formed of a carbide. The first layer further includes ferrite, or may further include a mixture of pearlite and ferrite.

When the specimen of the present Example is compared with an SCM420 specimen in which only martensite is formed in a surface layer by a rotating bending test at 1200 MPa, the number of repeated fractures of the specimen of the present Example is 50 times larger. As such, it was confirmed in the fatigue test that the lifespan is prolonged. The structural material in the present Example can be applied to a member to which a long lifespan is essential as a sliding material, and particularly, can be used in components such as torque transmission components, repeating operation units, or rotating units for automobiles.

The constitution required for securing a fatigue life as in the present Example is represented in the following.

(1) The structural material has two carbides. A first carbide is a compound having TiC as a main component, and a second carbide is a compound mainly having M₃C (wherein M is Ti, Cr, or Mo), and the structural material has at least two carbides.

(2) The first carbide is formed as a layered form on the outermost surface of the structural material and a TiCN-based compound or a TiN-based compound is included in the portion thereof.

(3) The Vickers hardness of VC, VCN, or VN is in a range of 1500 to 3500.

(4) A thickness of the first carbide is in a range of 10 μm or more and 200 μm or less.

(5) For securing close adhesion between the second layer and the third layer, at least one of the constituent elements of the first carbide has a concentration gradient toward a center part, while at least one of the constituent elements diffused into the first layer and the second layer grows as a compound in the grain boundary or in the grain.

The above constitution is further described. 1) The carbide is formed as at least two kinds of a compound having TiC as a main component and a carbide mainly having M₃C (wherein M is Ti, Cr, or Mo). Since Ti is applied on the surface and then a heat treatment process is initiated, Ti is in a large amount on the surface and in a small amount in the inside of the material. Since a diffusion distance of Ti is about 100 μm at 1100° C., a concentration of Ti becomes about 10:1 on the surface of the base material and in the depth of 100 μm. In the vicinity of the surface of SCM420 as the base material, the concentration of Ti is greatly changed, and thus, a volume ratio of TiC and TiCN-based compounds is decreased. At a depth of 10 μm from the surface of the base material, M₃C as well as TiC and TiCN, is confirmed. M in M₃C contains Cr, Mo, or Ti.

A distance in a depth direction from which the concentration gradient of Ti is confirmed is in a range of 10 μm to 200 μm. The concentration gradient can be confirmed by composition analysis of a section by a scanning electron microscope, Auger analysis, or the like. By forming the concentration gradient of Ti, close adhesion between a base material and a TiC-based layered film can be secured, whereby the fatigue life is improved by 50 times longer than the case without TiC, by the TiC-based film which is not peeled off at 2000 MPa.

Though Ti is applied in the present Example, VC, NbC, MoC, TaC, or WC is formed by applying V, Nb, Mo, Ta, or W, and a carbide in which these elements are substituted for M in M₃C is formed in the inside of the base material.

2) A carbide having TiC as a main component is in a layered form on the outermost surface, some TiCN-based or TiN-based compounds are confirmed, and a volume ratio in the layered carbide becomes TiC>TiCN>TiN.

3) The Vickers hardness of TiC, TiCN, or TiN is in a range of 1500 to 3500, and the thickness thereof is 10 μm or more and 200 μm or less. When the Vickers hardness of the layered carbide is less than 1500, cracks easily occur, so that an effect of improving a lifespan in the rotating bending test is not significant. In the rotating bending test at 2000 MPa, when the case in which only martensite is formed is defined as 1, the case in which carburization is performed by controlling a determined concentration or texture without using Ti becomes 5, and the case in which a TiC-based layered carbide is formed after Ti is applied and at the same time a Ti diffusion layer is formed for securing close adhesion becomes 10 to 50. For the dramatic improvement of lifespan, the Vickers hardness of the layered carbide (such as TiC) or the layered carbonitride (such as TiCN), or the layered nitride (such as TiN) needs to be 1500 or more, and for a 50 times longer lifespan, the Vickers hardness of 2500 to 3000 is preferred.

4) For securing close adhesion of the surface layer carbide, at least one of the constituent elements of the surface layer carbide has a concentration gradient of 0.5%/μm to 10%/μm for Ti toward the center part of the base material. In addition, Ti which is diffused into SCM420 grows as a compound in the grain boundary or in the grain, and some form cementite referred to as M₃C.

In addition to the improved lifespan in the rotating bending fatigue test, an effect of improving a lifespan is confirmed also in a pitching test in a roller test. The same effect can be confirmed for the base material other than SCM420 without limitation of the composition as long as the base material is versatile FeC-based alloy steel such as low alloy steel or case hardening steel.

Example 3

On a surface of an SCM420 material (0.2 wt % of C, 0.3% of Si, 0.7% of Mn, 0.1% of Cu, 0.1% of Ni, 1.1% of Cr, 0.2% of Mo, and the balance of Fe), V powder is mixed at 10 wt % in a methanol solvent and pulverized by a ball mill. The V powder is in a range of 0.01 μm to 10 μm, and a slurry formed of methanol and Ti powder is formed. This slurry is applied on the SCM420 material and dried, and the thickness of the applied film is 50 μm. The SCM420 material on which the V applied film is formed is inserted to a carburizing furnace, and heated to 1200° C. By heating to 1200° C., counter diffusion between V and SCM420 proceeds, so that V is diffused into the grain boundary or into the grain of SCM420 and partially becomes VC. After V is diffused, the temperature is lowered to 1050° C. and acetylene is introduced. An amount of introduced acetylene depends on a depth and a texture requiring hardness (amount of carbide or the like); however, up to 1.2 wt % of carbon is introduced in a γ region. By the primary carburization, martensite is partially grown in the vicinity of the surface layer of SCM420 by argon gas cooling. At this cooling rate, carbides more than 10 μm do not grow in the grain boundary of SCM420.

After cooling, heating is maintained at 700° C., which is just below an A1 point (codeposition transformation point), for 1 hour and then heating is performed to a temperature of the A1 point or more, and a carbide is precipitated in the γ crystal grain. From the precipitated carbide, carbon is diffused to the inside of SCM420, and acetylene is introduced for suppressing a decrease in a carbon concentration, thereby diffusing carbon from the surface. Due to the introduction of acetylene, the carbide is grown, so that a granular carbide is dispersed and grown in a γ crystal grain boundary or into the grain, and the particle diameter becomes 0.5 to 2 μm. Here, carbon is diffused into SCM420 through VC.

The texture of the structural material manufactured by the process has a three-layer structure in order from the center to the surface in a depth direction. A first layer is formed of pearlite, a second layer is formed of a mixture of martensite and a carbide, and a third layer is formed of a carbide. The first layer further includes ferrite, or may further include a mixture of pearlite and ferrite.

When the specimen of the present Example is compared with an SCM420 specimen in which only martensite is formed in a surface layer by a rotating bending test at 1200 MPa, the number of repeated fractures of the specimen of the present Example is 50 times larger. As such, it was confirmed in the fatigue test that the lifespan is prolonged. The structural material in the present Example can be applied to a member to which a long lifespan is essential as a sliding material, and particularly, can be used in components such as torque transmission components, repeating operation units, or rotating units for automobiles.

The constitution required for securing a fatigue life as in the present Example is represented in the following.

(1) The structural material has two carbides. A first carbide is a compound having TiC as a main component, and a second carbide is a compound mainly having M₃C (wherein M is Ti, Cr, or Mo), and the structural material has at least two carbides.

(2) The first carbide is formed as a layered form on the outermost surface of the structural material and a VCN-based compound or a VN-based compound is included in the portion thereof.

(3) The Vickers hardness of VC, VCN, or VN is in a range of 1500 to 3500.

(4) A thickness of the first carbide is in a range of 10 μm or more and 200 μm or less.

(5) For securing close adhesion between the second layer and the third layer, at least one of the constituent elements of the first carbide has a concentration gradient toward a center part, while at least one of the constituent elements diffused into the first layer and the second layer grows as a compound in the grain boundary or in the grain.

The above constitution is further described. 1) The carbide is formed as at least two kinds of a compound having VC as a main component and a carbide mainly having M₃C (wherein M is V, Cr, or Mo). Since V is applied on the surface and then a heat treatment process is initiated, V is in a large amount on the surface and in a small amount in the inside of the material. Since a diffusion distance of V is about 100 μm at 1100° C., a concentration of V becomes about 10:1 on the surface of the base material and in the depth of 100 μm. In the vicinity of the surface of SCM420 as the base material, the concentration of V is greatly changed, and thus, a volume ratio of VC and VCN-based compounds is decreased. At a depth of 10 μm from the surface of the base material, M₃C as well as VC and VCN, is confirmed. M in M₃C contains Cr, Mo, or V.

A distance in a depth direction from which the concentration gradient of V is confirmed is in a range of 10 μm to 200 μm. The concentration gradient can be confirmed by composition analysis of a section by a scanning electron microscope, Auger analysis, or the like. By forming the concentration gradient of Ti, close adhesion between a base material and a VC-based layered film can be secured, whereby the fatigue life is improved by 50 times longer than the case without VC, by the VC-based film which is not peeled off at 2000 MPa.

Though V is applied in the present Example, an MC compound such as TiC, NbC, MoC, TaC, WC, or (Ti, Cr) C is formed by applying at least one or more of Ti, Nb, Mo, Ta, W, or Cr, and a carbide in which these elements are substituted for M in M₃C is formed in the inside of the base material.

2) A carbide having VC as a main component is in a layered form on the outermost surface, some VCN-based or VN-based compounds are confirmed, and a volume ratio in the layered carbide becomes VC>VCN>VN.

3) The Vickers hardness of VC, VCN, or VN is in a range of 1500 to 3500, and the thickness thereof is 10 μm or more and 200 μm or less. When the Vickers hardness of the layered carbide is less than 1500, cracks easily occur, so that an effect of improving a lifespan in the rotating bending test is not significant. In the rotating bending test at 2000 MPa, when the case in which only martensite is formed is defined as 1, the case in which carburization is performed by controlling a determined concentration or texture without using V becomes 5, and the case in which a VC-based layered carbide is formed after V is applied and at the same time a V diffusion layer is formed for securing close adhesion becomes 10 to 50. For the dramatic improvement of lifespan, the Vickers hardness of the layered carbide (such as VC) or the layered carbonitride (such as VCN), or the layered nitride (such as VN) needs to be 1500 or more, and for a 50 times longer lifespan, the Vickers hardness of 2500 to 3000 is preferred.

4) For securing close adhesion of the surface layer carbide, at least one of the constituent elements of the surface layer carbide has a concentration gradient of 0.5%/μm to 10%/μm for V toward the center part of the base material. In addition, V which is diffused into SCM420 grows as a compound in the grain boundary or in the grain, and some form cementite referred to as M₃C.

In addition to the improved lifespan in the rotating bending fatigue test, an effect of improving a lifespan is confirmed also in a pitching test in a roller test. The same effect can be confirmed for the base material other than SCM420 without limitation of the composition as long as the base material is versatile FeC-based alloy steel such as low alloy steel or case hardening steel.

Example 4

Ti is pulverized in a methanol solvent to form an amorphous form having a Ti particle diameter of 1 to 50 μm. Methanol containing 10 wt % of Ti pulverized powder is applied on the surface of an SCM420 material (0.2 wt % of C, 0.3% of Si, 0.7% of Mn, 0.1% of Cu, 0.1% of Ni, 1.1% of Cr, 0.2% of Mo, and the balance of Fe) in the air. The thickness of Ti after application is 20 μm. Methanol as a solvent is evaporated and dried, and inserted in a carburizing furnace. The inside of the carburizing furnace is vacuum-exhausted to 1 Pa and then substituted with Ar to clean the atmosphere in the furnace.

The partial pressure of H₂O and O₂ is decreased and whether the time change is minimized is confirmed, and then heating is started. Heating is performed to 1100° C. at a rate of 10° C./min and the temperature is maintained for 1 hour, and acetylene (C₂H₂) gas is introduced. By the introduction of the acetylene gas, Ti and a part of Ti diffused from the surface of SCM420 are carbonized, and carbon is diffused into the surface of SCM420. A carbon concentration is surface Ti>SCM420 surface, and Ti becomes TiC or TiCN. Ti is diffused at a temperature of 1100° C. and diffused into the grain boundary or into the grains of SCM420. A part of the diffused Ti forms TiC or (Ti, Cr, Mo)₃C, by diffusion of carbon due to the introduction of acetylene. The TiC does not continuously grow in the γ grain boundary of SCM420, and grows in a granular form. A total carbon supply amount at 1050° C. is 1.2 wt % in the SCM420 side of the TiC/SCM420 interface.

TiC/SCM420 carburized at 1100° C. is reheated to 850° C. and carburized again by acetylene gas. A total carbon supply amount at 850° C. is smaller than the supply amount at 1050° C., and the concentration of carbon in the SCM420 side of the TiC/SCM420 interface is 1.4 wt %. By the carburization, a spherical carbide is grown in the grain. N₂ gas is introduced during the carburization process to perform nitrification after carburization, and oil quenching and tempering are performed. An oil temperature is 100° C. and a tempering temperature is 160° C.

The texture of the structural material manufactured by the process is a layered carbide/a mixture of carbide and martensite/pearlite/a mixed phase of ferrite and pearlite, in a depth direction from the surface to the center part. A thickness of layered carbide is 20 μm as confirmed in the photograph of the section in FIG. 1. The part of a mixed phase of martensite and spherical carbide is in a range to 40 to 50 μm from the interface between the layered carbide and the base material. In this range, a concentration gradient of Ti is confirmed. A carbon concentration in the surface layer part is 12 to 17 wt %, but a carbon concentration in the mixed phase of martensite and carbide is 1.0 to 1.7 wt %, and a concentration of Ti forming the carbide is decreased. The concentration gradient of Ti is 1 wt %/μm in the mixed phase of martensite and carbide, and the concentration gradient is increased to the vicinity of the layered carbide.

As compared with the case of carburization quenching and tempering without forming a carbide, a fatigue life measured by a rotating bending fatigue test is 20 times longer in the present Example. A factor causing the high fatigue life is represented below. TiC or TiCN is formed on the surface at a thickness of 20 μm and the hardness is 1550 to 1800 Hv, as shown in FIG. 2. The TiC or TiCN and TiNO can be confirmed by X-ray diffraction patterns as shown in FIG. 3, and the region in which carbide is dispersed in SCM420 is formed between SCM420 and a TiC-based film, thereby increasing close adhesion of TiC or TiCN. Around the granular TiC, martensite and residual austenite are formed, and the hardness of martensite is 750 Hv. Granular TiC is not confirmed in the center of the material, and grown by diffusion of Ti and carbon, and an amount of TiC is decreased from the surface to a center part, and as the carbide, cementite which is Fe₃C-based is increased rather than TiC-based. By having the constitution, a high-hardness TiC or TiCN-based film which is difficult to be abraded is formed in the surface layer, and occurrence and propagation of cracks are suppressed by dispersed high-hardness TiC and fine martensite around the high-hardness TiC, and Fe₃C and martensite around the high-hardness TiC.

Example 5

The section schematic diagram of the present Example is shown in FIG. 4. A layer including MC carbide is formed in the outside of SCM420 as a base material. A mixed phase of M₃C carbide and martensite is formed in the base material side of MC carbide (1). In FIG. 4, M₃C carbide (3) having a spherical shape can be seen and is dispersed in martensite (2). Pearlite is formed in the further inner side (inner side) of the mixed phase. M represented by MC carbide contains at least one of Ti, Nb, Ta, V, W, and Mo, and an M element forming the MC carbide is contained also in M₃C. Fe in the M₃C carbide has a higher concentration than Fe in the MC carbide.

When the specimen of the present Example is compared with an SCM420 specimen in which only martensite is formed in a surface layer by a rotating bending test at 1200 MPa, the number of repeated fractures of the specimen of the present Example is 50 times larger. As such, it was confirmed that the lifespan in the fatigue test was prolonged, and thus, the treatment process and the material constitution in the present Example can be applied to a member to which a long lifespan is essential as a sliding material, and particularly, can be used in components such as torque transmission components, repeating operation units, or rotating units for automobiles.

The improvement of the fatigue life can be achieved by having the texture constitution of FIG. 4 also in an iron carbon-based structural material other than SCM420.

Example 6

The results of the fatigue test are shown in FIG. 5. Upon comparison with gas carburization, a surface pressure at the same fatigue cycle is increased 1.3 times. In addition, at the same surface pressure, the lifespan is increased 10 times or more. As such, it was confirmed that the lifespan in the fatigue test was prolonged, and thus, the treatment process and the material constitution can be applied to a member to which a long lifespan is essential as a sliding material, and particularly, can be used in components such as torque transmission components, repeating operation units, or rotating units for automobiles.

In the present embodiment, MC is present in the outermost surface, and M₃C is present in martensite. Since MC has a higher hardness than M₃C, and has excellent abrasion resistance, MC is needed in the outermost surface layer. A carbon concentration is lowered in the inside to disperse M₃C.

REFERENCE SIGNS LIST

-   1 MC carbide -   2 Martensite -   3 M₃C carbide -   4 Pearlite 

1. A structural material containing iron and carbon, comprising: a first layer formed of pearlite, a second layer formed of a mixed phase of martensite and a carbide, and a third layer formed of a carbide, in order from a center to a surface of the structural material, wherein the carbide of the third layer is represented by MC (wherein M is one element among Ti, V, Nb, Mo, Ta, and W), and the structural material has a concentration gradient in which an M element concentration is decreased from the surface to the center.
 2. The structural material according to claim 1, wherein the carbide of the second layer is represented by M₃C (wherein M is one element among Ti, V, Nb, Mo, Ta, and W), and in the second layer, a spherical carbide is dispersed in the martensite to form the mixed phase.
 3. The structural material according to claim 1, wherein the third layer contains a carbonitride.
 4. The structural material according to claim 1, wherein the third layer has a Vickers hardness in a range of 1500 to
 3500. 